Integrated circuits have evolved into complex devices that can include millions of transistors, capacitors, and resistors on a single chip. In the course of integrated circuit evolution, functional density (i.e., the number of interconnected devices per chip area) has generally increased while geometry size (i.e., the smallest component (or line) that can be created using a fabrication process) has decreased.
Transistors are circuit components or elements that are often formed on semiconductor devices. Many transistors may be formed on a semiconductor device in addition to capacitors, inductors, resistors, diodes, conductive lines, or other elements, depending on the circuit design. Integrated circuits incorporate planar field-effect transistors (FETs) in which current flows through a semiconducting channel between a source and a drain, in response to a voltage applied to a control gate.
As device dimensions have shrunk, device geometries and materials have experienced difficulty maintaining switching speeds without incurring failures. Several new technologies emerged that allowed chip designers to continue shrinking gate lengths. One particularly far-reaching technology change entailed re-designing the structure of the FET from a planar device to a three-dimensional device in which the semiconducting channel was replaced by a fin that extends out from the plane of the substrate. In such a device, commonly referred to as a FinFET, the control gate wraps around three sides of the fin so as to influence current flow from three surfaces instead of one. The improved control achieved with a 3-D design results in faster switching performance and reduced current leakage. Building taller devices has also permitted increasing the device density within the same footprint that had previously been occupied by a planar FET.
The FinFET concept was extended by development of a gate all-around FET (GAA FET), in which the gate fully wraps around the channel for maximum control of the current flow therein. In the GAA FET, the channel can take the form of a cylindrical nanowire that is isolated from the substrate. Existing GAA FETs are oriented horizontally, such that the nanowire extends in a direction that is parallel to the surface of the semiconductor substrate.
The FinFET concept was further extended by development of a vertical gate all-around FET (vGAA FET), in which a current-carrying nanowire is oriented perpendicular to the surface of the semiconductor substrate. In terms of scaling, however, the amount of area used in a vGAA FET is prohibitive. Additionally, neither GAA nor vGAA eliminate current leakage. Thus, there is a need for gate all-around FETs that have improved performance for transistors having critical dimensions below 7 nm.